The memory of a computer is often subjected to a hostile environment: neutrons, gamma radiation, heat, shock, and the like. It is important to know how well the memory has retained its information. It is therefore important to be able to test the memory after it has failed. The tester, however, should not be in the hostile environment. The partially damaged memory is therefore removed from the hostile environment and plugged into the tester in a benign environment.
This test method is not entirely satisfactory. For one thing, it is necessary for some one or some thing to enter the hostile environment in order to remove the memory, and this is, by definition, unsafe. For another, the tester can determine which bits of the memory have undergone permanent damage, but temporary damage evaluation is beyond its capability. For a third, the tester is unable to determine exactly what the status of each of a fairly large number of bits is, microsecond by microsecond, as an environmental assault (such as, for example, a shock wave) passes through the memory.
It is possible to place the tester at a remote location, so that it will not be subjected to the same hostile environment or assault, and to test the memory as the assault passes through the memory. However, a long cable must then be extended from the memory to the tester, and this causes problems in itself. The output signals of the typical computer memory were never intended to be sent over great distances, but only to the next chip, or to the next circuit board at the most. The output signals are therefore degraded in and by the cable, and the tester cannot extract as much meaningful information as is desired.